REGULATION OF TRANSPIRATION IN COFFEE HEDGEROWS - COVARIATION OF ENVIRONMENTAL VARIABLES AND APPARENT RESPONSES OF STOMATA TO WIND AND HUMIDITY

Stomatal regulation of transpiration was studied in hedgerow coffee (C offea arabica L.) at different stages of canopy development encompassi ng a range of leaf area indices (L) from 0.7 to 6.7, Stomatal (g(s)) a nd crown (g(c)) conductance attained maximum values early during the d ay and then declined as both leaf-to-bulk air water vapour mole fracti on difference (V-a) and photosynthetically active photon flux density (I) continued to increase, Covariation of environmental variables duri ng the day, particularly V, I, and wind speed (u), obscured stomatal r esponses Co individual variables, This also caused diurnal hysteresis in the relationship between g(c) and individual variables, Normalizati on of g(s) and g(c) by I removed the hysteresis and revealed a strong stomatal response to humidity, At the crown scale, transpiration (E) i ncreased linearly with net radiation (R(n)) and seemed to increase wit h increasing wind speed, Increasing wind speed imposed higher leaf int erior to leaf surface water vapour mole fraction differences (V-s) at given levels of V-a. However, strong relationships between declining g (c) and E and increasing wind speed were obtained when g(c) and E were normalized by I and R(n), respectively, without invoking additional p otential interactions involving temperature or CO2 concentration at th e leaf surface, Apparent stomatal responses to wind were thus at least partially a reflection of the stomatal response to humidity.